Methods for forming a thin film on a surface of an object using a gas-phase state in which a substance is in a state capable of moving on an atomic/molecular level in a manner similar to gas can be largely classified into chemical vapor deposition (CVD) and physical vapor deposition (PVD).
Representative methods of PVD can include a vacuum deposition method, a sputtering method, and the like. In particular, although the apparatus cost is high, the sputtering method is generally capable of forming a high-quality thin film having excellent uniformity in terms of film quality and film thickness, and the sputtering method is widely applied to display devices such as liquid crystal displays, and the like.
In addition, CVD is a method in which a starting material gas is introduced into a vacuum chamber, a single type or two or more types of gas are decomposed or reacted on a substrate using thermal energy, and a solid-state thin film is thereby grown. Here, there are also methods in which plasma or catalytic reactions are concomitantly used to promote reaction during film formation and to reduce reaction temperature. These methods are respectively referred to as plasma enhanced CVD (PECVD), Cat-CVD, and the like. Such methods of CVD are primarily applied to the manufacturing processes for semiconductor devices, such as formation of a gate insulator film, because there are few film formation defects.
Furthermore, atomic layer deposition (ALD) has been gaining attention in recent years. The ALD method is a method in which film formation of a substance adsorbed on a surface is performed a layer at a time on an atomic level by chemical reaction being induced on the surface. The ALD method is classified within CVD. The ALD method and typical CVD are differentiated by the following points. Typical CVD is a method in which a thin film is formed by a single gas or a plurality of gases being used at the same time and reacted on a substrate. Instead, the ALD method alternately uses a high activity gas, referred to as a precursor (such as tri-methyl aluminum (TMA)), and a reactive gas (the reactive gas may also be referred to as a precursor in the ALD method). Therefore, the ALD method is a special film formation method in which a thin film is grown a layer at a time on an atomic level through adsorption on a substrate surface and chemical reaction following adsorption.
A specific film formation method of the ALD method will be described hereafter. The ALD method is a film formation method that takes advantage of a phenomenon that occurs in surface adsorption on a substrate, that is, a so-called self-limiting effect, in which, after the surface is covered by a certain type of gas, no further adsorption of the gas occurs. As a result of the self-limiting effect, after only a single layer of the precursor (first precursor) is adsorbed on the substrate, the unreacted precursor is discharged. Next, the reactive gas (second precursor) is introduced onto the substrate, and only a single layer of a thin film having a desired composition is deposited on the substrate by oxidizing or reducing the precursor. The reactive gas is then discharged. The above-described process comprises a single cycle, and the thin film is grown by repetition of this cycle. Therefore, in the ALD method, the thin film grows in a two dimensional manner. In addition, the ALD method is known to produce fewer film formation defects compared to the conventional vacuum deposition method, sputtering method, and the like. Moreover, the ALD method is known to produce fewer film formation defects even compared to typical CVD and the like.
Therefore, application of the ALD method to a wide range of fields, such as the packaging industry for food products, pharmaceutical products, and the like, and the electronic component industry, is anticipated.
In addition, in the ALD method, a method is known in which plasma is used to activate a reaction in the process for decomposing the second precursor and reacting the second precursor with the first precursor that is adsorbed on the substrate. This method is referred to as plasma enhanced ALD (PEALD) or simply plasma ALD.
The technique of the ALD method itself was proposed in 1974 by Dr. Tuomo Sumtola of Finland. Because the ALD method generally enables formation of high-quality, high-density thin films, application of the ALD method in the semiconductor industry, such as for gate insulator films, is advancing. The ALD method is disclosed in the International Technology Roadmap for Semiconductors (ITRS). Furthermore, the ALD method is known to not produce a slanted shadow effect (a phenomenon in which variations in film formation occur as a result of sputtering particles being incident on the substrate surface at an angle) compared to other film formation methods. Therefore, in the ALD method, a precursor can be applied to a substrate or the like as long as a gap allowing infiltration of gas is present. Therefore, application of the ALD method is also anticipated in uses related to microelectromechanical systems (MEMS) for the purpose of coating three dimensional structures, in addition to coating lines and holes on a substrate that has a high aspect ratio in which the ratio of depth and width is large.
However, the ALD method also has the following disadvantages. Specifically, special materials are used to perform the ALD method, so the cost increases as a result of the use of special materials, and the like. Moreover, the greatest disadvantage of the ALD method is that the film formation speed is low. For example, in the ALD method, the film formation speed is lower by about 5 to 10 times compared to film formation methods such as typical vacuum deposition, sputtering, and the like.
There are various materials that serve as targets on which thin films are formed by the ALD method using the film formation method such as that described above. For example, as thin-film formation targets for the ALD method, there are small, plate-shaped substrates such as wafers and photomasks, inflexible substrates having a large surface area such as glass plates, flexible substrates having a large surface area such as films, and the like. Corresponding to the intended uses of such substrates, various methods for handling these substrates are being proposed and put into practical use in mass production facilities for forming thin films on these substrates, from the perspective of cost, ease-of-handling, film formation quality, and the like.
For example, in an example in which a film is formed on a wafer, a single-wafer film formation apparatus is known in which a single substrate is fed into the film formation apparatus and a film is formed. Thereafter, the substrate is replaced with the next substrate and film formation is performed again. A batch-type film formation apparatus is also known in which a plurality of substrates are collectively set and the same film formation is performed on all wafers.
In addition, in an example in which film formation is performed on a glass substrate or the like, an inline-type film formation apparatus is known in which film formation is simultaneously performed while substrates are successively carried to a section serving as the source of film formation. Furthermore, mainly in the case of flexible substrates, a so-called roll-to-roll web coating film formation apparatus is known, which is a method in which a substrate is unwound from a roll, film formation is performed while the substrate is being carried, and the substrate is wound onto another roll. The latter also includes a web coating film formation apparatus in which, not only flexible substrates, but also substrates subject to film formation are placed on a flexible sheet or a partially flexible tray that is capable of continuously carrying the substrate, and continuous film formation is performed.
In the film formation methods and substrate handling methods of any of the film formation apparatuses, a combination of film formation apparatuses that have the highest film formation speed is used through determination based on cost, quality, ease-of-handling, and the like.
As related technology, a technique is disclosed in which a gas permeation barrier layer is formed on a plastic substrate or a glass substrate by atomic layer deposition being performed on the plastic substrate or the glass substrate by the ALD method (for example, refer to PTL 1). In this technique, a light-emitting polymer is mounted on a plastic substrate that has flexibility and light transmittance, and atomic layer deposition is performed by the ALD method on the top surface and side surfaces of the light-emitting polymer (top-coating is performed). It is known that, as a result, coating defects can be reduced and a light-transmissive barrier film that is capable of significantly reducing gas permeation with a thickness of several tens of nanometers can be actualized.
In addition, in recent years, as products obtained through the ALD method, products for the purpose of imparting flexibility and reducing weight that are related to the back sheet and front sheet of solar cells, displays and lighting such as organic electroluminescent (EL) elements, and the like, and products, regardless of field, that are provided with high-barrier films of 10−3 g/(m2·day) or lower and serve as glass substrate substitutes are desired. Furthermore, in high-barrier films, barrier films that are resistant to temperatures and moisture are desired.
Ordinarily, in a gas barrier film, a metal coating or a metal oxide coating is formed on at least one surface (first surface) of a base material using the CVD method, sputtering method, or sol-gel method. However, when heat resistance and moisture resistance of the base material is low, various problems occur when a base material having a high thermal expansion coefficient (also referred to, in particular, as a linear expansion rate or a linear expansion coefficient), a base material having a low glass transition point, and the like are used. For example, when a product using such a base material is exposed to stress, such as heat, during reliability testing of the product, the metal coating or the metal oxide coating formed on the base material may deteriorate as a result of expansion, contraction, and deformation, and defects may increase. Therefore, a substrate using a base material such as those described above may not be able to maintain the desired gas barrier properties.
Therefore, in light of problems such as those described above, for example, a gas barrier film has been proposed in which a metal oxide film that serves as the gas barrier film is formed by the CVD film or the sol-gel method on a base material having a low thermal expansion rate, thereby imparting resistance to temperatures and moisture (for example, refer to PTL 2). However, in the gas barrier film formed on the base material by the CVD method or the sol-gel method, only a water vapor transmission rate (WVTR) in the 10−2 g/(m2·day) order can be obtained. Therefore, the desired high-barrier film is difficult to obtain.
In addition, as described above, because the thin film formed by the ALD method grows in a two-dimensional manner and has fewer film formation defects compared to the CVD method, the sputtering method, and the like, obtainment of a high-barrier film can be anticipated. Furthermore, when film formation is performed by the ALD method, because the precursor, such as an organic metal, attaches to the functional groups or the like on the base material surface, the state of the base material surface on which the film is formed is also important in obtaining a thin film with few film formation defects. Therefore, adsorption of the precursor, such as an organic metal, on the base material surface is required to be effectively performed and a base material having a low linear expansion coefficient is required to be selected. Furthermore, a layer structure that is provided with the desired characteristics is required to be formed.